Yes, it only takes just
8 Notes for a US district court to consider in regards to plagiarism. For example; the case in regards to George Harrison - My Sweet Lord. Did this case set a condition for which all future cases are determined and should the law catch up to expand the limitations to 16 notes due to immense quantity and nature of the beast?
My answer; I really don't know and I guess if you go by Sylvester Stallone's famous line in the movie Judge Dredd he would say... "
the law is the law"
RE: yeah, yeah, yeah, I know, I know, but
I guess most cases determine by decree; 'unintentional use' which then falls under the lesser outcome usually resulting in a 50/50 split (more or less) of all revenue/income past, present and future. So yes; unintentional use is not going to save you.
That said; there are already a lot of songs that all sound similar and use the same chord progressions along with a very similar melody. For example; during a TV commercial break I heard two jingles from two totally different products (adverts) aired consecutively use the exact same chords and very similar melody. In fact I hear this sort of thing almost every day.
I know for a fact in my own dealings with a publisher; they will toss 'ideas' around among their writers in regards 'direction', 'style' and 'sounds' they want to model on what in their opinion is the 'now' happening thang. Often at meetings they'll talk about; yeah Miss Doris your song has great potential but that beat and other elements of the song needs more work = take these demo CD's home and study them carefully, listen to how Mr.Jon Doe constructs and try to adapt those ideas to your songs.
Fact is 'in regards to intellectual rights'; you cannot copyright
ideas but of course there is a limit to how much (in length) a writer can use in regards to 'The Melody' which effectively sets the course in regards to all cases for example; they simply detach those 8 notes (melody) to sound on it's own 'apart from the accompaniment and harmonies' to then determine the final verdict; thereby literally the final score!
I believe almost all writers respect other writer(s) work (aww heck I hope I'm right about that). I also believe in the case with George Harrison and many others; in their case it was
unintentional use. Seriously, there are so many writers in this crazy world it would not surprise me that some writers from totally different countries and cultures are actually working on the very same melody at this very moment in time.